The present invention pertains to a process for the conversion of a paraffinic feedstock, in particular for the conversion of a paraffinic feedstock derived from a Fischer-Tropsch synthesis process.
In a Fischer-Tropsch process, synthesis gas is fed into a reactor where it is converted over a suitable catalyst at elevated temperature and pressure to paraffinic compounds ranging from methane to high molecular weight modules comprising up to 200 carbon atoms, or, under particular circumstances, even more. Synthesis gas or syn gas is a mixture of hydrogen and carbon monoxide that is obtained by conversion of a hydrocarbonaceous feedstock. Suitable feedstock include natural gas, crude oil, heavy oil fractions, coal, biomass and lignite. Processes to convert the hydrocarbon feedstock to synthesis gas include gasification, steam reforming, auto-thermal reforming and (catalytic) partial oxidation.
While the products obtained in a Fischer-Tropsch synthesis have attractive properties, for example low levels of contaminants like sulphur and nitrogen, they generally have a too high melting point to be directly suitable for general use as liquid fuels or lubricants. Therefore, especially the higher-boiling fractions are generally subjected to an upgrading step. The upgrading step is intended to effect one or more of a decrease in viscosity, a decrease in pour point or cloud point, and a decrease in (end) boiling point.
In the art, products obtained from a Fischer-Tropsch process are often subjected to a hydrocracking step followed by a fractionation step. One or more boiling point fractions of the hydrocracked product can be subjected to a dewaxing step.
There is need for improvement of this process, and the present invention provides such an improved process. More in particular for certain applications there is a need to lower the aromatics content of the products and for other applications there is a need to increase the yield of intermediate product (waxy raffinate). This will be further explained in the text below.